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353

between serving as a bath to maintain the temperature constant Ther-
mometers c and d give the temperatures in the vessel A and in the bath
and the latter * heated by means of a ring burner ; the whole is supported
on a tnpod. Under the efflux tube is a glass flask C with marks on the
neck at 200 c.c. and 240 c.c. All
the parts of the apparatus are of
exactly standardised form and dimen-
sions.

To make  a  determination,   the
internal vessel is filled to the desired

level with the oil to   be examined
(dehydrated   by   decantation    and

filtered through cotton wool dried

at   100°),  the   efflux   orifice   being

shut.    The outer vessel is then filled

with  water   and   heated   carefully

until the oil reaches the temperature

at  which  the  viscosity   is   to   be

measured.   The flask is then placed

underneath   and   the plug  rapidly

withdrawn, the time being  counted

exactly   from    this    instant.    The

FIG. 48

exact time taken to fill the flask to the 200 c.c. mark, divided by that
taken under similar conditions, with a standard liquid, gives the viscosity
of the oil in Engler degrees.

Usually a temperature of 20° is employed and water taken as the standard,
and the apparatus should be controlled from time to time with water; as
a rule 52-53 seconds are required for the efflux of 200 c.c. of water.

Between one determination and another of the time of efflux of water, the
difference is _+ 1-5 second, which corresponds with a difference of + 3% in
the degrees Engler. If, then, the difference in the time of efflux of water is
less than i sec., as prescribed for the use of the apparatus, the difference in the
degrees Engler is _+ 2-3%.

Each apparatus is sold duly controlled, the time of efflux for water being
indicated.

For very dense heavy oils, and for such semi-solid products as vaseline,
the viscosity should be determined at a higher temperature, e.g., 50°, 60°,
100° or, sometimes, 180° or even higher. In the latter cases the whole
of the apparatus is placed in a large oven, or use made of an apparatus in
which the outer vessel—tightly closed and fitted with a reflux condenser
—serves as a vapour-bath in which water (100°), aniline (180°) or other
suitable liquid is boiled. In some cases it may be advantageous to determine
the viscosity at temperatures below 20°.

Prior to a determination, the apparatus should always be thoroughly
cleaned from every trace of oil by washing with benzine or ether and drying
with absorbent paper.

The viscosity of heavy oils constitutes an indirect indication of their
A.c.                                                                                          23d the apparatus heated to 80°, after